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I was reading the introductory chapter of my textbook on machines and when the author comes to a little introduction on synchronous machines, I was stuck on a statement midway. It goes:

"When balanced 3-phase currents are allowed to flow in the armature winding, these produce a synchronously rotating field, stationary with respect to rotor field as a result of which the machine produces torque of electromagnetic origin."

First of all, the book follows an approach of never mentioning motor or generator it mentions just 'machine' which is fair enough cos the schematic diagrams of motor and generator are the same with reversed energy flows.

But my question here is:

That in my amateur opinion electromagnetic torque is produced when there is an interaction between two magnetic fields with a relative speed between them. If they are stationary with respect to each other, how can a torque be generated? Please clarify.

P.S: if you could also explain separately the cases of motor and generator.

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Well, you cannot deny there is a force between rotor and stator. It's exactly the same force between two fixed permanent magnets. They are fixed but if let loose would collide with each other. This can't happen in a motor because the rotor is constrained radially. Does torque also exist? Yes it does - to overcome windage and friction exhibited by the rotor. That's the no load scenario and clearly an increase in torque exists when mechanical load is applied.

It's also interesting to note that if the rotor was mechanically lossless, the rotating mag field would be physically coincident with the exact centre of the rotor pole aligned with it. As load is applied there becomes a misalignment that inevitably produces a hight force between rotor and rotating field. Maybe this is a more satisfactory explanation?

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For an asynchronous machine, you need relative motion between the stator and the rotor because that is how current is induced in the rotor. If you have zero relative speed between the 2, then no current is induced in the rotor and then there will be no magnetic field in the rotor.

However, for synchronous machines, the field in the rotor is not induced by the stator field but is separate from it (due to a separately excited winding or permanent magnets). So if the rotor has its own magnetic field that doesn't depend on the stator, then the goal (generally) is to keep those fields separated by a constant angle that maximizes the torque (generally 90 degrees).

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The book does not differentiate between motor and generator because a synchronous machine will behave as either. If you apply a load to the shaft, opposing the direction of rotation, it will act as a motor, drawing power from the line. Conversely, if you apply torque in the direction of rotation, as if trying to speed up the shaft, it will behave as a generator; power will flow from the machine to the line.

If you were to take a synchronous motor, disconnected from the line (the field excited, if not permanent magnet) and spin the shaft, you will observe an AC voltage at the terminals. This is a result of the rotor field moving past the stator windings: Faraday's law. This voltage is referred to as the back emf of the motor. The motor is a generator. This voltage is generated whenever the motor is rotating, in particular, when it is connected to the line and operating normally.

Suppose your motor is running unloaded. In this scenario, the rotor field aligns with the rotating stator field in such a position that the line voltage, the voltage drops in the stator, and the back emf induced in the stator are in balance. Now you apply some load torque to the shaft. This extra torque causes a small displacement in the rotor-stator alignment, so that the induced back emf is now lagging a bit. This unbalances the stator voltage, causing a (torque producing) current to flow. This additional current brings the stator voltages back into balance, and the machine settles into a stable operation at a new rotor displacement. Generator operation is identical, except that torque, displacement and current are in the opposite direction.

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